Users of mobile communication devices, such as cellular telephones, Personal Digital Assistants (PDAs) or laptop computers that include wireless capability, etc., often experience performance problems, such as dropped calls, poor call quality, and an inability to connect with the network. Such problems are often the result of interference from other wireless signals in the area. Additionally, however, such problems are often the result of what is called multipath interference.
The term multipath is a term that describes how a signal transmitted in a wireless environment travels along multiple paths from the transmission source to the destination or receiver. For example, when a base station transmits a signal to a mobile communication device, the energy comprising the signal spreads out. Some of the energy can travel along a direct line to the mobile communication device. This direct line is one path. Some of the energy can, e.g., reflect off a building and then reach the mobile communication device. The reflected signal path being a second path. Similarly, some of the energy can reflect off other buildings, mountains, or other structures before reaching the mobile communication device. The different paths traveled by the signal energy from the base station to the mobile device are referred to as multipaths, and the associated signal energies are referred to as the multipath signals, or sometimes multipath for short.
The multipath signals combine with each other in the mobile communication device receiver. At times the multipath signals will combine constructively, but at other times the signals will combine destructively, i.e., the signals will combine in such a manner that they at least partially cancel each other out, or interfere with each other. This is because the multipath signals can be out of phase with each other due to the different lengths of the paths traveled. Destructive multipath combining, or interference, can lower the signal-to-noise ratio in the receiver, and affect other signal parameters, causing the problems referred to above. Such destructive multipath interference is often referred to as fading, i.e., it causes the signal as seen by the mobile communication device receiver to fade out.
Spatial diversity has been used to combat the problem of destructive multipath interference, or fading. In its simplest form, spatial diversity comprises two antennas spaced a certain distance apart. The distance between the antennas should be related to the wavelength of the signal being received, e.g., a multiple or sub-multiple of the wavelength. The idea of spatial diversity is that the distance between the antennas allows each antenna to receive samples of the signal independent of the other antenna. While the signal at one antenna might be experiencing destructive interference, the signal at the other might be experiencing constructive interference.
The difference in position of the antennas will affect the phase of the multipath signals. The effect of the different path lengths can affect the phase of the multipath signals enough such that multipath signals that would have combined destructively at the first antenna, will now combine constructively at the second antenna. Thus, spatial diversity can improve performance and help overcome, e.g., the problems referred to above. Moreover, spatial diversity can extend to any number of antennas.
A mobile communication device can, therefore, be configured with a plurality of antennas and a means for changing between antennas when the received signal quality is degraded beyond a certain point, which can for example be measured in terms of received signal power. Accordingly, the mobile communication device can be configured to monitor the signal power of a signal received using a first antenna of a plurality of antennas. When the received signal power drops below a certain threshold, then the device can be configured to switch to another antenna that exhibits higher received signal power.
Since mobile communications devices are typically not large enough to implement true spatial diversity, polarization diversity can be implemented in order to improve performance in a mobile communication device. The polarization diversity case is similar to the spatial diversity case. Whereas spatial diversity relies on the separation of the antenna to get independent samples, polarization diversity relies on the different polarizations. For example a vertically polarized antenna will tend to see vertically polarized signals and tend to reject horizontally polarized signals; therefore, samples from a vertically polarized antenna will tend to be independent from samples from a horizontally polarized antenna. Spatial diversity can also be combined with polarization diversity, as in the case where a vertically polarized antenna and a horizontally polarized antenna are included in the same device. Because the two antennas are typically located at different locations within the device, they will exhibit at least some degree of spatial diversity.
Thus, a plurality of antennas can be incorporated into a communication device that comprises spatial diversity, polarization diversity, or both, such that the device can switch between different antennas and/or different polarizations in order to attempt to improve the received signal quality.
A smart antenna system is an antenna system that is capable of steering the antenna beam or is capable of beam forming. Examples of types of smart antennas would include a single active element with parasitic elements. By modifying the characteristics of the parasitic elements the beam can be steered, shaped, or both. Another example smart antenna can include multiple active elements where the phase of the signal between the elements can be changed to cause the beam to steer or change shape. Alternatively, a smart antenna can include multiple active elements that allow the signals to be applied to each independently and weighted to steer or form the beam. Processing for these smart antennas can, for example, be done in DSP.
In conventional devices, the device must check each antenna to determine if there is an antenna with better signal quality than the current configuration. Unfortunately, this can actually degrade device performance even further, since often many if not all of the other configuration will have worse signal quality than the current configuration. Thus, the device can spend significant time searching configuration that actually have worse performance than the current configuration, which degrades the device's overall performance during the searching period.